The design of extraction, fractionation and purification processes of natural products requires the existence of thermodynamic models that are able to accurately represent the phase equilibria behaviour of the highly complex mixtures involved in these unit operations. The complexity of these multi-component mixtures results from several reasons, e.g., the significant differences in size between the recovered natural products and the corresponding solvents and the existence of specific interactions, like hydrogen-bonding.
The main purpose of this work is the development of thermodynamic models that take into account explicitly the self-association and/or solvation effects between molecules and its subsequent
application to natural products mixtures. A compilation on the association theories, its analogies and limitations is presented. The application to multi-component mixtures, for which the available experimental information is scarce, suggests the use of group-contribution methods. Therefore, in this
study, an association contribution based on the first order perturbation theory was developed following a group-contribution approach.
The phase equilibria predictions for these multi-components systems usually are made using an equation of state or an activity coefficient model. Two new models were developed, one for the calculation of activity coefficients (A-UNIFAC) and one equation of state for the calculation of fugacity coefficients (GCA-EoS).
The A-UNIFAC model results from the addition of one association term to the combinatorial and residual contributions from the original UNIFAC model. First, the interaction parameters of the model were obtained by correlating vapour-liquid equilibria (VLE), liquid-liquid equilibria (LLE) and infinite
dilution activity coefficients of binary mixtures containing water, alcohols, acids, esters, aromatic compounds, alkyl chlorides and/or alkanes. Significant improvements were obtained relatively to the
original UNIFAC model for the prediction of VLE and, specially, for the prediction of infinite dilution activity coefficients of associating components in inerts. After this, the model was used to predict the
solid-liquid equilibria (SLE) and VLE of mono and disaccharides in polar solvents (water and alcohols).
For that, new UNIFAC groups were defined to represent sugar molecules. The interaction parameters for these new groups were obtained using VLE and SLE of sugar-solvent binary mixtures. Finally, the model was applied to multi-component mixtures of industrial interest, for example, for the prediction of the water activity (in honey or fruit juices) or for the prediction of SLE of sugars in mixed solvents. This
model is quite satisfactory in describing the non-idealities in the liquid phase in a wide range of compositions.
The equation for the fugacity coefficients calculation (the group-contribution with association equation of state, GCA-EoS) combines three terms: a repulsive term based on the hard-spheres theory, a NRTL group version for the attractive term and, finally, the group-contribution association term based on the first order perturbation theory. The association term has been revised to allow the extension of
the model to several associating components, namely, carboxylic acids, esters, ketones in mixtures with water, alcohols and any number of inerts. The results obtained with the GCA -EoS are very satisfactory. These results compare favourably with the predictions of another group-contribution model that does not take into account explicitly association effects (MHV2 model), specially for
mixtures of associating components with inerts.
The fugacity coefficients calculation for the liquid and vapour phases using an equation of state allows its application up to the critical region. The GCA-EoS was used to describe phase equilibria of mixtures containing fatty oils (triglycerides) and their derivatives (fatty acids, fatty acids esters, mono and diglycerides) with supercritical fluids (propane or carbon dioxide). The critical parameters of high
molecular weight components are derived from infinite dilution activity coefficient data. New interaction parameters were calculated between the acid and the triglyceride group. As a complement to the available experimental information, infinite dilution activity coefficients for several solutes in mixtures of a triglyceride and a fatty acid were measured by inverse gas chromatography.
The GCA equation of state has shown a great flexibility to represent this type of mixtures containing polar mixtures with asymmetric components which supports its application in the design and optimization of separation processes of fatty oils mixtures with supercritical fluids.